Epithelial cells rely on the polarized distribution of their transmembrane proteins for functional interactions with adjacent cells, the extracellular matrix, growth factors, and most other components of their external environment. Thus, an elucidation of the mechanisms giving rise to membrane polarity is important to the understanding of epithelial function. This proposal seeks to examine the role of PDZ proteins in the polarized distribution of transmembrane proteins which bear carboxy-terminal PDZ-binding peptide motifs. Preliminary experiments, described herein, provide strong evidence that polarization of syndecan-1, a heparan sulfate proteoglycan, depends upon the interaction of its cytoplasmic tail with a PDZ protein in Madin Darby Canine Kidney (MDCK) cells. After confirming and quantifying these results, I wish to investigate how general a mechanism this phenomenon is. First, I will test the abilities of various alternative C-terminal PDZ-binding motifs to direct the polarization of syndecan-1. Next, I will examine the abilities of different PDZ proteins to specify syndecan-1 localization. This will be accomplished through co-expression of syndecan-1 with various recombinant PDZ proteins which are able to bind syndecan's C-terminal motif but are differentially targeted to the cytoplasm, apical, or basolateral plasma membrane domains.